A typical nuclear power plant includes high pressure and low pressure turbines which drive generators producing the plant's electrical output, the steam for the high pressure turbine being provided via the output of the steam generating reactor installation. The steam goes first to the high pressure turbine and via an intermediate superheater to the low pressure turbine.
Some of the steam generated by the nuclear installation supplies the intermediate superheater to raise the temperature of the steam leaving the high pressure turbine and going to the low pressure turbine.
Such an intermediate superheater sometimes comprises a vertical casing having a bottom provided with an inlet for the steam leaving the high pressure turbine, and a top providing an outlet for the steam going to the low pressure turbine. One or more heat exchangers are positioned inside of this casing, each comprising a bundle of upstanding tubes which may be vertical or inclined, the tubes having an inlet manifold for their upper ends and an outlet manifold for their lower ends. The inlet manifold has an inlet for the steam from the nuclear installation and the outlet manifold has an outlet for condensate.
When operating as intended to superheat the steam passing through the casing for use by the low pressure turbine, the steam entering the inlet manifold of the heat exchanger should flow downwardly in a uniform manner through all of the tubes of the bundle of tubes, with the steam giving up all of its heat to the steam going to the low pressure turbine, the steam ultimately completely condensing at or near the tube bottoms and being withdrawn from the outlet manifold as a water condensate.
Unfortunately, generally the operation of such an intermediate superheater involves the problem that the steam completely condenses in some of the tubes while passing freely through others of the tubes so as to exhaust into the outlet manifold and be lost via the condensate discharge. In addition, the tube or tubes where complete condensation has occurred becomes blocked by the pressure of the freely passed steam exhausting into the outlet manifold, the blocked water introducing the possiblilty of tube corrosion at an unexpected rate. If the steam supplied to the intermediate supereheater's heat exchanger includes a non-condensable gas, such as air, such a gas also has a tendency to collect in some of the tubes so as to decrease the heat efficiency desired. In other words, it is possible for both water and air to collect in those parts of the bundle of tubes where stationary or static conditions exist.
It is possible to avoid the above indicated problem, by increasing the steam flow from the reactor installation through the bundle of tubes of the intermediate superheater. With enough steam water and non-condensable gases can be flushed and kept flushed out of the bundle of tubes. This has the disadvantage that the surplus of steam is discharged through the condensate drain of the heat exchanger's oulet manifold, so that the heat content of this surplus steam is lost.